1. Field of the Invention
The invention relates to a steering system.
2. Discussion of Background
In a conventional vehicle steering system, the rotation of a steering shaft caused by steering of a steering wheel is transmitted to a steered mechanism such as a rack-and-pinion mechanism. Thus, the angle of steered wheels is changed. A rack end is provided at each shaft end portion of a rack shaft that constitutes the above-described rack-and-pinion mechanism. The rack end connects a tie rod to the rack shaft such that the tie rod is pivotable relative to the rack shaft. When the steering wheel is steered, a steering input that acts on the rack shaft is transmitted to the steered wheels via the rack ends and the tie rods.
In the steering system that includes the above-described steered mechanism, when the rack end comes into contact with a rack housing that accommodates the rack shaft, the movement of the rack shaft is mechanically restricted. In other words, the contact of the rack end with the rack housing occurs, whereby a movable range of the rack shaft and a movable range of the steered wheels are determined.
In the meantime, if the steered wheel is turned, for example, when the steered wheel runs onto a curbstone, an input (reverse input) due to the turning of the steered wheel acts on the rack end. Furthermore, if the reverse input is greater than the steering input caused by a driver's steering operation, there is a possibility that the rack end may be moved to collide with the rack housing due to the reverse input and a strong impact may act on a steering system.
In recent years, there has been developed a steering system in which an impact absorbing member made of an elastic material is arranged between the rack housing and each rack end to absorb an impact load when a collision occurs (for example, WO 2005/123484 A1).
In the steering system that includes the above-described steered mechanism, even when a driver steers the steering wheel until the steering angle reaches a maximum allowable steering angle, the contact of the rack end with the rack housing occurs. That is, in the above-described steering system, even when the contact of the rack end with the rack housing occurs due to a steering input caused by the driver, the impact absorbing member arranged between the rack end and the rack housing is elastically deformed. Therefore, when the driver steers the steering wheel until the steering angle becomes a steering angle close to the allowable steering angle, the driver feels a sense of discomfort due to elasticity caused by the deformation of the impact absorbing member.
When the steering input is changed under such a situation, the deformation amount of the impact absorbing member changes. Therefore, the distance between the rack end and the rack housing is changed. That is, even a slight change in the steering input causes a change in the angle of the steered wheels. Therefore, for example, when the driver turns a vehicle while maintaining the state where the steering wheel has been fully steered and kept at the allowable steering angle, the steering input needs to be maintained constant. Therefore, the drivability may be deteriorated.